The present invention relates to a sheath heater serving as a heating element in a sheath type glow plug used to preheat a sub combustion chamber or a combustion chamber of a diesel engine and a method of manufacturing the same and, more particularly, to improvements in a connection structure between a sheath and a coil-like heating wire (to be referred to as a heater coil hereinafter) and a method of connecting the sheath and the heater coil.
In general, since starting properties at a low temperature are poor in a diesel engine, a glow plug is provided in a sub combustion chamber or a combustion chamber, and a current is flowed to the glow plug to generate heat, thereby improving the ignition properties of the engine by increasing an intake air temperature or using the glow plug as an ignition source. An example of a conventional glow plug of this type is a sheath type glow plug as disclosed in, e.g., Japanese Patent Laid-Open No. 57-58017 or Japanese Patent Publication No. 63-64690. This conventional sheath type glow plug includes a sheath heater in which a heater coil consisting of an iron-chromium alloy or the like (or a combination of a heater coil and a resistive element consisting of, e.g., pure nickel having a large positive resistance temperature coefficient and connected in series with the heater coil as a power control element) is inserted in a sheath consisting of a heat-resistant metal material such as stainless steel such that one end of the heater coil is connected to the distal end portion of the sheath and the other end of the heater coil is connected to the distal end of an electrode rod inserted from the distal end portion of the sheath, and a heat-resistant insulating powder such as magnesia is charged in the sheath.
To fix the heater coil at the sheath distal end portion in the conventional sheath type glow plug having the above arrangement, a cap as an independent member to be fitted in and integrated with an opening at the distal end portion of the sheath is generally used, as disclosed in Japanese Patent Laid-Open No. 58-21. However, a fitting of this cap in the sheath distal end portion is cumbersome, and the workability of spot-welding the heater coil to the cap is poor. Therefore, it is difficult to automatically assemble such a sheath heater. In addition, this conventional structure is complicated because the number of constituting parts is large, and the manufacture of the structure is cumbersome because the heater coil must be formed to have a to-be-welded end extending from the distal end of the coil and to be welded to the cap.
In addition, Japanese Patent Publication No 42-15989 or West Germany Patent No. 3,003,799 discloses a well-known conventional sheath heater having a structure in which, as shown in FIG. 6A, a small hole 1a is formed in the distal end portion of a sheath 1 as described above, the distal end portion of a heater coil 2 is inserted into the small hole la from inside to project outside the sheath 1, and this projecting portion is melted and connected by welding. However, this sheath heater poses problems in durability in that, for example, a connection portion 3 (FIG. 6C) obtained by this connecting method is broken by oxidation after repetitive use of the glow plug.
The present inventor has made extensive studies and researches on the problem of durability in the connection portion 3 of the above conventional sheath heater and found that in the connection portion 3 formed at the distal end portion of the sheath 1 by welding the sheath 1 and the heater coil 2 according to the above conventional connecting method, as shown in FIGS. 6B and 7A, an unnecessarily large amount of a heater coil member consisting of an iron-based material is fused and mixed in a surface layer portion of a stainless steel member constituting the sheath 1 to form a fused portion 4 (the content of the coil member is large especially in a portion 4a shown in FIG. 7A), and these fused portions 4 and 4a between the sheath and coil members are easily oxidized in a relatively short period of time when a high-temperature state is repeatedly caused by heat generated upon application of power to the glow plug. This oxidation is gradually promoted, and the oxidized portions are broken when the high-temperature state is repeated 20,000 to 30,000 times. As a result, holes are formed as shown in FIGS. 6C and 7B, thereby degrading the durability. That is, since a large amount of the iron-based material of the coil 2 is mixed in the stainless steel member of the sheath 1 at the above fused portions 4 and 4a between the sheath 1 and the coil 2, the concentration of Ni contained in the sheath member is decreased to degrade the essential durability of the stainless steel member. Therefore, a certain countermeasure is required to solve these problems.
In particular, a strong demand has recently arisen for a so-called afterglow system in which a power supply state to a glow plug is maintained for a predetermined time period after an engine is started, thereby smoothly and properly performing combustion inside the engine. In addition, the operation time of this afterglow system is desired to be prolonged, and various operating conditions such as the heat generation temperature must be more precisely controlled. Therefore, a countermeasure against the above problem of durability caused by oxidation must be made in consideration of these conditions.